A measuring while drilling apparatus for making porosity, density and other formation characteristic measurements is described in U.S. Pat. No. 4,879,463 issued Nov. 7, 1989 and assigned to the assignee of the present invention. The patent describes a drilling collar in which two radioactive sources are provided. A neutron source is positioned near the cylindrical axis of the tool while a gamma ray source is eccentered against an interior side of the collar's cylindrical body. Both sources can be axially inserted into and removed from the body's interior via one end of the body. Secondary radiation detectors for the porosity measurement are provided within an interior cylindrical body secured within the collar cylindrical body. Similarly, gamma radiation detectors are arranged within the interior body.
Stabilizer blades provided about the outer radius of the cylindrical tool aid in the drilling process. Openings in a blade angularly aligned with the gamma radiation detectors are aligned with openings in the steel cylindrical body which are also aligned with the detectors. Radiation transparent materials are provided in the openings of the steel body and the adjacent stabilizer blade.
The apparatus described above represents significant advances in the field of performing porosity and neutron measurements of surrounding formations while drilling a borehole. First, the nuclear sources are placed within the body of the collar on a retrievable carrier which is loaded into the collar from its end. When inserted, the gamma ray source automatically is properly placed in an eccentered position in the collar; the neutron source is placed on the center-line of the collar. Advantageously, if the drill collar were to become stuck in the hole, a fishing head placed at the top of the carrier may be latched by means of fishing equipment such as a wireline-conveyed overshot through the center or mud flow path of the drill string. The carrier with both nuclear sources may then be brought to the surface.
Performing porosity measurements and density measurements while drilling results in certain advantages over conventional wireline porosity and density measurements. Longer sample periods due to the slower nature of the drilling process reduce the statistical variations and uncertainty in measuring while drilling porosity and density measurements. Many of the borehole effects that perturb wireline measurements of porosity or density are reduced because the drill collar substantially fills the borehole while drilling. Also, formation effects, lithology and salinity changes under drilling conditions are comparable to or less than those for an open hole wireline measurement which may occur hours or even days after the borehole is drilled. However, the washing action of drilling fluid while drilling can produce variations in borehole size. Increased variations in borehole diameter are called washouts. Separation, or "standoff", of the tool from the borehole wall causes measured data perturbations. The occurrence of washouts exacerbates the standoff effect.
The apparatus of U.S. Pat. No. 4,879,463 described above performs well under ordinary drilling conditions. For example, where an eight and one half inch (81/2") drill bit is used, a six and one half inch (61/2") drill collar is used above it. With the detectors within the collar cylindrical body, approximately a one inch standoff exists between the tool and the borehole wall. However, where larger size holes are drilled, for example with a twelve and one-quarter inch (121/4") drill bit, an eight inch (8") drill collar is typically used above it. The combination of such a 121/4" bit and an 8" collar results in a nominal two inch standoff between the tool and the borehole wall. Such large standoffs are disadvantageous as explained above.
One measuring while drilling assembly is schematically illustrated in an advertisement brochure of Gearhart Geodata Services. As best can be understood from the schematic illustrations, a radiation source and near and far detectors are placed in one of four stabilizer fins of a cylindrical body of a MWD porosity tool. Except for the diameter of the device, its physical characteristics appear to be essentially the same as a conventional wireline compensated neutron porosity tool in that a conventional compensated neutron porosity tool is designed to be run eccentered in the borehole. In other words, a single source, near detector, and far detector eccentered alignment appears to be contemplated in the proposed Gearhart Geodata Services device with a stabilizer blade serving as the mechanism for providing eccentering of the source and detectors.
A similar measurement while drilling neutron porosity tool is described in a brochure of TELECO OILFIELD SERVICES, INC. bearing a copyright notice of 1990 with a further notation of May 1990. The brochure describes a similar drill collar with a source and a single pair of near and far detectors aligned with the source. The tool includes a 63/4" diameter mandrel with a 71/2" upset. Three fluted channels located in the upset allow for return mud circulation. The source and detectors are aligned with one of the three resulting "vanes" but apparently are not placed radially beyond the nominal 63/4" diameter.